In general, as shown in FIG. 1, according to an electro-discharge machining scheme employing an electrode wire 2 for electro-discharge machining, the electrode wire 2 is inserted into a workpiece 1 through a start hole 7 which has been previously perforated through the workpiece 1. A high-frequency voltage is applied between the electrode wire 2 and an inner wall surface of the start hole 7 while the electrode wire 2 is being continuously inserted into the workpiece 1 in the perforation direction of the start hole 7, thereby generating arc between the electrode wire 2 and the inner wall surface of the start hole 7, so that the workpiece 1 is melted. Then, melts are removed by using a machining liquid and the instantaneous vaporization power between the electrode wire 2 and the workpiece 1, so that the workpiece 1 can be machined in a desired shape.
According to the electro-discharge machining principle, an electro-discharge machine includes a power supply, a wire transferring unit for electro-discharge machining, a workpiece transferring unit, and a machining liquid circulating unit.
In general, as indicated by the arrow in FIG. 1, the workpiece transferring unit moves perpendicularly to the electrode wire 2 for electro-discharge machining. After the electrode wire 2 for electro-discharge machining continuously reeled out of a supply reel 3 is hang on guide rollers 5 and 5′ provided at both end portions of the workpiece 1, the electrode wire 2 is wound around a winding reel 4.
In this case, a high frequency-voltage is applied between the workpiece 1 and the electrode wire 2 for electro-discharge machining through the power supply 6 to perform a cutting machining process, and deionized water is supplied to a machining region as a machining liquid in order to discharge heat emitted in the cutting machining process. The efficiency of the electro-discharge machining, especially, the machining speed, significantly depends on machining parameters such as the feeding speed of the machining liquid, machining current density, and the shape and frequency of the machining voltage, and the efficiency of the electro-discharge machining can be improved by adjusting the machining parameters.
Pure copper has been used in a conventional technology since the pure copper has high electrical conductivity and facilities a fine wire process due to a high elongation property. However, since a pure copper line represents a low tensile strength in the electro-discharge machining, the pure copper may be easily disconnected. In addition, a high tensile strength cannot be applied to the copper line, so that vibration of the electrode wire 2 cannot be controlled, thereby resulting in an inferior machining accuracy.
In addition, the copper wire represents a relatively slower machining speed. Therefore, a high strength wire such as a molybdenum wire or a tungsten wire has been used for a special application of high machining precision. In addition, a brass electrode wire, such as a brass wire including copper and zinc in a weight ratio of 65%:35%, has been developed for the general purpose of wire electro-discharge machining.
When comparing with a pure copper wire, the brass electrode wire has a tensile strength which is at least twice greater than the tensile strength of the copper wire, and more improves discharge stability and instantaneous vaporization power due to zinc which is an alloy component of the brass electrode wire. Accordingly, when comparing with the pure copper wire, the brass electrode wire improves the machining speed and the machining precision.
In addition, as the electro-discharge machining scheme has been extensively used, the demand for the improvement of a tensile strength and the machining speed is increased. Accordingly, an advanced brass electrode wire has been developed by adding a small amount of Al, Si, and the like to the brass electrode wire, so that the tensile strength and the machining speed of the brass electrode wire can be improved.
Meanwhile, as zinc content is increased in a brass alloy, the machining speed may be increased. However, if the zinc content is more than 40 weight % in the brass alloy, a weak brittle phase β is formed, so that a drawing process may be difficult when a fine wire is formed.
In order to solve the above problem, the inventor of the present invention has suggested the structure of an electrode wire in Korea Patent Registration No. 10-518727, in which the electrode wire includes a core wire including a first metal including copper, an alloy layer, which is formed from an outer portion of the core wire toward the center of the core wire by diffusing the component of a second metal to the first metal through the mutual diffusion reaction between the first and second metals at the boundary region of the core wire, an alloy plated layer, which is formed on the core wire by diffusing the component of the first metal to the second metal through the mutual diffusion reaction between the first and second metals, and a plating layer, which is formed on the alloy plated layer and includes the second metal having a vaporization temperature lower than that of the first metal constituting the core wire. In this case, the alloy plated layer is formed on the core wire through the mutual diffusion reaction between the first and second metals, so that the alloy plated layer represents the highest hardness and the lowest elongate percentage among the layers. In addition, the alloy plated layer and the plating layer have cracks appearing perpendicularly to the longitudinal direction of the electrode wire.
In addition, the inventor of the present invention has suggested the structure of an electrode wire including a core wire including a first metal including copper, an alloy layer, which is formed from an outer portion of the core wire toward the center of the core wire by diffusing the component of a second metal to the first metal through the mutual diffusion reaction between the first and second metals at the boundary region of the core wire, and an alloy plated layer, which is formed on the core wire by diffusing the component of the first metal to the second metal through the mutual diffusion reaction between the first and second metals. In this case, the alloy plated layer is formed on the core wire through the mutual diffusion reaction between the first metal and the second metal having a vaporization temperature lower than that of the first metal to represent the hardness higher than that of the core wire and the elongation percentage lower than that of the core wire. The alloy plated layer has cracks appearing perpendicularly to the longitudinal direction of the electrode wire. The first metal includes copper, brass, or a copper alloy, and the second metal includes zinc, aluminum, tin, or the alloy thereof.
Further, the inventor of the present invention has suggested a method for manufacturing an electrode wire for electro-discharge machining in Korea Patent Registration No. 10-518731, in which the method includes preparing an intermediate wire rode, which serves as a core wire, includes a first metal including copper, and has a first diameter, forming an alloy layer, which represents the hardness higher than those of the first and second metals and the elongation percentage lower than those of the first and second metals, on an outer portion of the core wire through the mutual diffusion reaction between the first and second metals by passing the core wire including the first metal through a plating bath containing the second metal melted therein and having a vaporization temperature lower than that of the first metal and forming a plating layer including the second metal on the alloy layer, allowing cracks to appear on the alloy layer and the plating layer due to the higher hardness and the lower elongation percentage of the alloy layer by drawing the intermediate wire rod having the alloy and plating layers so that the intermediate wire rod has a second diameter, and stabilizing a mechanical characteristic of a fine wire by performing a heat treatment process with respect to the fine wire having the cracks.
In order to form the alloy and plating layers on the core wire, the core wire is passed through the plating bath for one second to ten seconds at a temperature of about 400° C. to about 500° C. The fist metal includes copper, brass, or a copper alloy, and the second metal includes zinc, aluminum, tin or the alloy thereof.
In addition, the inventor of the present invention has suggested a method for manufacturing an electrode wire for electro-discharge machining in Korea Patent Registration No. 10-518733, in which the method includes preparing an intermediate wire rode, which serves as a core wire, includes a first metal including copper, and has a first diameter, forming an alloy plated layer, which represents the hardness higher than those of the first and second metals and the elongation percentage lower than those of the first and second metals, on an outer portion of the core wire through the mutual diffusion reaction between the first and second metals by passing the core wire including the first metal through a plating bath containing the second metal melted therein and having a vaporization temperature lower than that of the first metal, allowing cracks to appear on the alloy plated layer due to the higher hardness and the lower elongation percentage of the alloy layer by drawing the intermediate wire rod having the alloy plated layer so that the intermediate wire rod has a second diameter, and stabilizing a mechanical characteristic of a fine wire by performing a heat treatment process with respect to the fine wire having the cracks.
According to the related arts, an electrode wire having an alloy layer including copper-zinc grain fragments is formed through the mutual diffusion reaction with a core wire metal including copper performed due to the melted zinc and applied heat, so that the machining speed can be improved. However, when performing an elongation process for a brass core wire representing 510N, the brass core wire is strengthened, so that an alloy plated layer provided at an outer portion of the core wire may be easily fragmented, thereby producing a great amount of micro-particles in electro-discharge machining.